Voltage-controlled oscillators (VCOs) are an essential constituent part of various control loops, for example phase-locked loops (PLLs). VCOs are often implemented by ring oscillators.
Typical functional characteristic variables of control loops comprise stability and dynamic response. Characteristic variables of this kind can be influenced, in particular, by the operating behavior of the VCOs. A significant characteristic variable of the voltage-controlled oscillators in this respect is the frequency characteristic curve. The frequency characteristic curve describes the change in frequency as a function of the control signal. In the case of VCOs, the frequency characteristic curve thus describes the change in frequency f as a function of the control voltage V: Δf/ΔV. Said frequency characteristic curve is often also referred to as KVCO. KVCO is typically dependent on the control voltage, KVCO=KVCO(V).
It may be necessary in the case of an excessively large KVCO—for example on average or as a maximum value—for the ring oscillator to be dimensioned to be larger in terms of surface area, in order to achieve a lower frequency. A signal-to-noise ratio can decrease overall because it may be necessary to change the manipulated variable of the control loop in a comparatively small range or to dimension the manipulated variable of the control loop to be very small in absolute terms. As a result, interfering influences become more significant.
FIG. 1A illustrates aspects in relation to a ring oscillator 100 known from the prior art, which implements a VCO having a comparatively steep frequency characteristic curve. The ring oscillator 100 comprises a chain 110 having an uneven number of inverters 111-113. The chain 110 is fed back via a corresponding branch 122. The ring oscillator is of annular design due to the design of the feedback chain 110.
The frequency behavior of the ring oscillator 100 is controlled by means of the control voltage Vtune, which is applied to a corresponding connection 121 and is passed on to all the inverters 111-113. The control voltage Vtune is used here as the supply voltage for the inverters 111-113.
FIG. 1B illustrates the frequency characteristic curve of the ring oscillator 100 from FIG. 1A. The dependency of the frequency on the control voltage Vtune is illustrated here on the left in FIG. 1B. The control voltage is varied here in the range of from 0.5 V to 1.5 V. The frequency varies in this range between approximately 20 MHz and 700 MHz. This corresponds to a large upstroke and therefore to a large KVCO.
The slope of the frequency characteristic curve, that is to say KVCO, is illustrated on the right in FIG. 1B. In particular, KVCO a dependency of KVCO on the control voltage Vtune is illustrated. It is clear from FIG. 1B that KVCO attains comparatively large values in the range of from approximately 300 MHz/V to 700 MHz/V. The maximum of 690 MHz/V corresponds to a frequency of 434 MHz.
Techniques for providing a ring oscillator having a flatter frequency characteristic curve are known from US 2009/0267698 A1. A parallel connection of a first inverter, which is driven by a first supply voltage having a variable value, and a second inverter, which is driven by a second supply voltage having a substantially fixed value, is implemented for this purpose.
A connection of this kind has various disadvantages. For example, in a connection of this kind, it may not be possible, or it may only be possible to a certain extent, to select the value of the supply voltage of the second inverter connected in parallel independently of the value of the supply voltage of the other inverter.